Liquid crystal display devices have been used for watches and electronic calculators, various measuring apparatuses, automotive panels, word processors, electronic notebooks, printers, computers, televisions, watches, advertising displays, etc. Typical examples of a liquid crystal display mode include a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a VA (vertical alignment) mode using TFT (thin-film transistor), an IPS (in-plane switching) mode, and the like. Liquid crystal compositions used for these liquid crystal display devices are required to have stability to external stimuli such as moisture, air, heat, light, and the like, exhibit a liquid crystal phase within as wide a temperature range as possible including room temperature as a center, and have low viscosity and low drive voltage. Further, each of the liquid crystal compositions is composed of several types to several tens types of compounds in order to have optimum values of dielectric anisotropy (Δε) and/or refractive index anisotropy (Δn) for a display device.
A vertical alignment-mode display uses a liquid crystal composition having negative Δε and is widely used for liquid crystal TV etc. On the other hand, low-voltage driving, fast response, and a wide operating temperature range are required for all driving methods. That is, a large absolute value of Δε, low viscosity (η), and a high nematic-isotropic liquid phase transition temperature (Tni) are required. Also, in view of setting of Δn×d which is the product of Δn and a cell gap (d), it is necessary to adjust Δn of a liquid crystal composition within a proper range according to the cell gap. In addition, when a liquid crystal display device is applied to a television or the like, fast response is regarded as important, and thus a liquid crystal composition having low γ1 is required. In particular, the necessity to decrease viscosity and, at the same time, increase Δn has recently arisen for decreasing a cell gap to achieve fast response. Therefore, Patent Literature 1 and Patent Literature 2 already disclose liquid crystal compositions using a compound having a fluorine-substituted ter-phenyl structure.
On the other hand, in order to practically use a liquid crystal composition for a liquid crystal display device, the liquid crystal composition is required to cause no defect in display quality. In particular, a liquid crystal composition used in an active matrix drive liquid crystal display device which is driven by TFT elements or the like is required to have high resistivity value or high voltage holding ratio. In addition, it becomes necessary to be stable for external stimuli such as light, heat, and the like. Accordingly, an antioxidant for improving stability for heat and a liquid crystal composition using the antioxidant are disclosed (refer to Patent Literature 3 and Patent Literature 4), but the stability is not necessarily satisfactory. In particular, liquid crystal compounds having large Δn have relatively low stability for light and heat, and thus quality stability of compositions is not satisfactory.
Further, with expanding application of liquid crystal display devices, significant changes are found in use method and manufacturing method thereof. In order to cope with these changes, it is demanded to optimize characteristics other than basic physical property values which have been known. That is, VA (vertical alignment)-mode and IPS (in-plane switching)-mode liquid crystal display devices using liquid crystal compositions are widely used, and supersized liquid crystal devices of 50 inches or more are put into practical application. With increases in substrate size, instead of a usual vacuum injection method, a one drop fill (ODF) method becomes the mainstream of a method of injecting a liquid crystal composition into a substrate (refer to Patent Literature 5). However, when a liquid crystal composition is dropped on a substrate, the problem of degrading display quality by dropping marks is surfaced. Further, for the purpose of forming a pre-tilt angle of a liquid crystal material in a liquid crystal display device and of achieving fast response, a PS liquid crystal display device (polymer stabilized) and a PSA liquid crystal display device (polymer sustained alignment) are developed (refer to Patent Literature 6), but the problem of dropping marks becomes a larger problem. That is, these display devices are characterized by adding a monomer to a liquid crystal composition and curing the monomer in the composition, and, in many cases, the monomer is cured by irradiating the composition with ultraviolet light. Therefore, when a component having low stability for light is added, a decrease in resistivity value or voltage holding ratio is caused and, in some causes, the occurrence of dropping marks is induced at the same time, thereby causing the problem of degrading the yield of liquid crystal display devices due to display defects.
Accordingly, there is demand for development of a liquid crystal display device having high stability to light, heat, and the like and causing little display defects such as image sticking, dropping marks, and the like while maintaining the characteristics and performance, such as fast response and the like, which are required for liquid crystal display devices. There is also demand for development of a liquid crystal display device causing little display defects such as image sticking, dropping marks, and the like even in a vertical alignment-mode display in which alignment is controlled by a general vertical alignment film, not in a mode in which a polymer layer is formed in a liquid crystal layer formed by polymerizing a polymerizable compound.